This invention relates generally to charged particle beams and is particularly directed to an electron beam focusing lens for use in cathode ray tubes (CRTs).
Electron guns employed in television CRTs are generally comprised of an electron beam source and an electron beam focus lens spatially oriented along the direction of travel of the electron beam. The electron beam source directs a beam of energetic electrons along a common axis, while the lens focuses the electron beam on the phosphor-bearing screen of the CRT. The typical focus lens makes use of electrostatic forces for controlling the path of the electrons and includes discrete, conductive, tubular elements arranged coaxially about the beam. Each of the conductive elements, or grids, is maintained at a predetermined voltage to establish the desired electrostatic focusing field. This focusing field is characterized generally as having an axial potential distribution which decreases smoothly and in some cases monotonically from a relatively intermediate potential to a relatively low potential spatially located at a lens intermediate position, and then increases smoothly from the relatively low potential to a relatively high potential as the CRT's phosphor bearing faceplate is approached.
The continuous, unitary electrostatic focusing field may be produced by various arrangements of focusing grids. One common prior art electron beam focusing arrangement is called the "bipotential lens" which is generally comprised of two electrodes for producing an axial potential distribution along the direction of travel of the electrons which increases monotonically from an initial low potential near the source to a final high potential. Unfortunately, the bipotential lens exhibits poor spherical aberration characteristics and poor electron beam spot size, particularly at high beam currents. The inability of an electron lens to focus the beam on the phosphor-bearing display screen to a small spot size results in significant loss in picture resolution.
Another class of lenses, termed the "unipotential lens," exhibits an axial potential distribution which is substantially saddle-shaped, with the potentials at the beginning and end of the lens substantially equal. The axial potential distribution in such lenses typically decreases monotonically from an initial relatively high potential near the electron source to a relatively low potential and then increases monotonically to a final, relatively high potential. This approach also suffers from limitations primarily in the form of arcing between its G.sub.2 and G.sub.3 grids which are closely spaced and maintained at a large potential difference.
Still another type of lens found in the prior art is the periodic extended field lens. While offering several advantages over the other prior art lenses discussed above, periodic lenses in general have been unable to overcome beam spot size limitations at high electron beam currents caused by space charge effects and magnification limitations particularly at low electron beam currents.
There are primarily three characteristics of an electrostatic focusing lens which determine the diameter, or spot size, of the electron beam incident upon the phosphorbearing display screen. These characteristics are its magnification, spherical aberration and space charge effect. It is desirable to minimize the magnification of the electrostatic focusing lens in order to reduce beam spot size. The magnification of the lens is an important factor in video image acuity at low electron beam currents, becoming less important at higher beam currents. Spherical aberration arises from the effect that the off axis rays experience a different focus strength which is proportional to the third power of the radius location of each ray. Spherical aberration only moderately affects video image acuity at low electron beam currents, becoming an increasingly important factor in the quality of the video image at higher beam currents. Space charge effect arises from the mutual repulsion of the negatively charged electrons. Space charge effect is a dominant factor in video image quality at high electron beam currents, becoming a less significant factor at lower beam currents. Table I summarizes the effects on electron beam spot size of the various aforementioned electrostatic focusing lens characteristics for both low and high electron beam currents.
TABLE I ______________________________________ Low Beam Current High Beam Current Spot Size Factor Performance Performance ______________________________________ Magnification Dominant Less Important Spherical Moderate Dominant Aberration Space Charge Not Important Important ______________________________________
The present invention overcomes the aforementioned limitations of the prior art by optimizing the aforementioned lens characteristics using an asymmetric unipotential electron beam focusing lens which allows for the formation of smaller electron beam spot sizes particularly at very low (100 microamps) and very high (5 milliamps) beam currents. The lens includes a pre-focus portion which applies an electrostatic field which fluctuates along the electron beam axis as the electrons enter the lens followed by an electrostatic field of increasing intensity as the electrons exit the lens for focusing the electron beam on a phosphorbearing screen. The asymmetric field effectively weakens the pre-focus electrostatic field for improved electron beam spot size particularly at very high and very low beam currents.